Celts have some nice pieces that are decent starters or excellent back-ups (i.e. Sully, Crowder, Zeller). To win a championship we need excellent starters (i.e. proven top players in their positions and top 3 drafts) so I would not want to trade a #1 overall pick that is likely to be a star in the league.
And I don't think Kings would take a #1 pick only for Cousins. It may be a steep price but all these player are good but not the level of starter we need for a championship team so we put in trade package and matches Cousins $14m salary plus we give Kings our #1 pick in 2016 (keep 2015 pick if it is #1 overall) and our first round pick from the Clippers this year and two second round picks.
Kings Get: Choice either Sully or Olynyk (both PF so we keep one), Zeller to fill the center position for them, Crowder (to back-up Rudy Gay), and Wallace for salary matching purposes (that is why we gave them a second first round pick).
Celts Get Cousins and rid themselves of Wallace. Here Celts roster prior to signing or trading for other FA:
C - Cousins
PF - Sully or Olynyk
SF - Young
SG - Bradley/Smart
PG - Turner/Pressey
This assumes Celts are going to trade or let expire all other higher veteran contracts (i.e. Thornton, Bass, Prince, etc.).
In this scenerio with Celts getting #1 overall pick they either take player or trade to get a top starter at either PG, SG, or SF. To fill out roster with plenty of cap space we go after Reggie Jackson to play PG and Jimmy Butler to play SG/SF.
For good measure sign KG for one year to mentor Cousin and Okafor
New Roster
C - Cousins/Okafor
PF - Sully or Olynyk/KG
SF - Butler/Young
SG - Bradley/Smart
PG - Jackson/Turner/Pressey